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Wednesday, July 20, 2011

Box Blight Disease of Buxus Plants

Buxus dieback

Plants of the genus Buxus (aka Box or Boxwood), which are often used for hedging and topiary, are being affected by a fungal disease caused by Cylidrocladium buxicola. It is thought it originated in Central America but C. boxicola was isolated from buxus hedging in the UK in the 1990s and has since spread to Europe and New Zealand. It causes dieback in the buxus, leaf drop and death in affected plants.

Affected dead leaves on buxus hedge

Early detection is vital. The initial infection is characterised by general darkening of the leaves, spreading in a circular fashion across the whole leaf. The infection then spreads to the stem below and the leaves die, turning light brown and falling off.

Complicating diagnosis, other die-back diseases, such as that caused by Phytopthera, cause similar symptoms, so it is not always easy to identify which disease is the cause. When affected, the buxus plants may also be susceptible to other infections such as mildew like fungi.

A recent study by Henricot et. al. of the Royal Horticultural Society, studied the control of C. buxicola using fungicides. This study found that among the fungicides tested carbendazim was effective at reducing and stopping growth of the fungus mycelium and mancozeb was effective at inhibiting the germination of spores. Kiwicare Super Spectrum (previously known as NO Insects or Disease Spectrum powder) contains a combination of these fungicides and is likely to provide some curative and preventative effect.

Although Henricot et al suggests they found little effect from copper spray I have had personal communication from professional landscape gardeners in New Zealand that they have used Kiwicare Root Protect Aliette with success, and Copper Oxy as a preventative, and it has worked to stop spread of disease when the disease has been identified soon enough. See here.

There are fewer options available to the home gardener in the UK than there are in New Zealand. So references that cite that 'there are no effective fungicides available for control of Box Blight' are referring to Europe not NZ.

It is also important to note that fungicide on its own may not be sufficient to control the disease once it has a hold. It is important to combine treatment with:

Improving air flow around the plants to reduce infection from wind borne spores.

Cutting out and destroying dead or affected material.

Proactively clearing up and destroying dead leaves and dead material from around the plants as the spores can remain viable in the leaves for some years.

It would also be advisable to sterilise clipping and cutting implements when trimming buxus so as not to spread the disease.

Other dieback diseases of buxus such as Phytopthera can be treated with the fungicide NO Root Rot Aliette. Phytopthera are root diseases and it is advised that roots as well as the upper parts of the plants are treated.

Gardening requires a lot of water - most of it in the form of perspiration.

That is a question I have not had before! I doubt that the beer itself would be likely to harm the plants, however, if the beer sits on the foliage for a period it is likely to be a substrate for disease such as sooty mould and may attract insects that could damage the shrubs. Hosing off the beer would be the best preventative action but if you get damage a treatment with a fungicide and/or insecticide may help.

I can't be sure what the 'worm' is in your buxus or whether it is the cause of the dieback, but my advice is to apply Kiwicare Super Spectrum which is both an insecticide and fungicide. I hope it will control the worm, which is likely to be an insect larva, and control buxus blight, which might the the cause of the dieback.

i assume you mean you have buxus topiary in a spherical shape? My advice would be the same for any affected buxus; prune out affected parts now. Ensure you use clean pruning implements and steles before moving between plants. Dispose of all trimmings. Then spray with fungicide congaing, thiram, mancozeb or carbendazim. I hope your buxus will recover and fill the missing parts.